The NMR spectroscopist often finds it necessary to observe a wide variety of nuclides, especially .sup.13 C, .sup.1 H, .sup.31 P, .sup.18 F, .sup.17 Al, .sup.29 Si, .sup.23 Na, .sup.17 O, and .sup.15 N in the study of commercially and scientifically important chemicals. It is often necessary, and always desirable, for the NMR sample spinner and the rest of the probe electronics to be devoid of the elements of interest to avoid interfering background signals. This generally necessitates the use of a wide variety of sample spinners of different materials. The materials selection is further severely limited by the requirement of very high strength, very high modulus, low dielectric loss, very low magnetic susceptibility, and chemical inertness with respect to the sample and gases present--all simultaneous requirements to be met at the temperature at which the NMR technique is to be applied to the desired sample.
Sample spinners heretofore available have not permitted convenient use at temperatures above 250.degree. C. The primary object of the instant invention is to allow convenient MAS NMR experiments at temperatures at least up to 650.degree. C. and perhaps up to 1200.degree. C. Another object is to obtain higher turbine efficiency for reduced drive gas heating problems and the laboratory hazards associated therewith. A further object is to make versatile sample containers available at an acceptable cost. A further object is to develop improved microturbines and thrust bearings for various applications.
FIG. 1 illustrates the prior art high-temperature MAS all-zirconia spinner assembly. A zirconia rotor 1, typically with outside diameter about 7 mm, has impulse turbine flutes 2 ground onto the outer surface at the drive end 3. A zirconia rotor plug 4 is cemented into the opposite end of the rotor. The sample is loaded into the rotor through a small axial hole 6. Centrifugal forces during spinning prevent the sample from escaping. Removal of the sample after the high temperature NMR experiment is usually very difficult, as some fusing or densification of the sample may have occurred at high temperature.
Air jets from nozzles 11 impinging on these flutes produce the desired spinning. Rotor surface speeds are typically 30% of the speed of sound. For example, a 7 mm spinner may rotate at 4500 Hz. Rear air bearing orifices 12 and front air bearing orifices 13 near opposite ends of the stator 14 provide radial support of the rotor within the zirconia stator 14. Air escaping rearward over the rotor plug 4 produces a radially-inward-flow axial thrust air bearing 16 between the end ring 17 and the rear end of the rotor. A manifold 18 inside the zirconia housing 19 distributes pressurized nitrogen to the bearing orifices 12, 13 and the drive nozzles 11. The receiver/transmitter copper coil 20 is wound around the stator with leads 21, 22 coming out through the housing. Nitrogen turbine exhaust gas 23 escapes through exhaust holes 24 in the housing cap 25 with sufficient backpressure to maintain axial positioning against the thrust air bearing 16.
The low efficiency of the impulse turbine with simple flutes 2 requires high nitrogen gas consumption. This in turn requires high heater power for high temperature operation. The hot exhaust gas 23 must be cooled to a safe temperature before it can be exhausted in the vicinity of the surrounding superconducting magnet required for the NMR experiment. This is typically achieved via a counterflow heat exchanger external to the spinner assembly. Space constraints inside the NMR magnet limit the extent to which this can be readily achieved. Hence, a reduction in gas consumption is highly beneficial.